To understand the influence of selective harvesting on
the adaptive capacity of fish populations in changing
environments, a computer simulation model was developed. In
the model, hypothetical populations were composed of five
different life histories, which were genetically determined.
Each life history type had its own rate of survival and
reproduction that varied with changing environment and
population density. In nonselective harvesting, life
history types were equally vulnerable to harvest. In
selective harvesting some life history types, were more
intensively harvested than others.
Population life history composition changed
continuously in response to both changing environmental
conditions and harvesting. Changes in life history
composition were closely related to changes in numerical
population performances such as density and yield.
In general, selective harvesting of the simulated
populations reduced mean and terminal abundance, total
catch, and life history diversity. Nonselective harvesting
tended to eliminate individuals evenly from the life history
distribution so that the life history diversity was not
greatly influenced. Selective harvesting tended to severely
reduce or eliminate the life history types that were more
intensively harvested. This generally resulted in extreme
reduction of life history diversity. Variation in
abundance, as measured by the absolute value of the
residuals, was consistently larger in selectively harvested
populations. Increased variation was probably related to
reduction in life history diversity. Perhaps the most
potentially alarming impact was the dramatic increase in
incidences of extinction of selectively harvested
populations at the highest exploitation rate. High
selective harvest rate led to severe reduction in life
history diversity which diminished the population's ability
to persist in a changing environment. Furthermore, the
extreme reduction of life history diversity made it
difficult for populations that were able to persist to
recover in abundance after termination of harvest.
It is important to note, however, that the above
conclusions are general outcomes based on 20 different
environmental regimes. The results from a few individual
regimes were not in accordance with some of the general
outcomes, illustrating simply that particular results are
highly contingent on the particular environmental pattern